In the fields of geophysics, and environmental and civil engineering, electrical ground surveys are conducted to map the resistivity and chargeability of a ground subsurface. Such surveys are sometimes referred to as ground resistivity surveys or ground induced polarization surveys. Soil resistivity measurements may be used in a sub-surface geophysical survey to identify ore locations, depth to bedrock and other geological phenomena.
Ground resistivity surveys typically involve injecting an electric current into the ground through a first electrode driven by a transmitter. The current in the ground is sourced through a second electrode inserted into the ground at some distance away from the first electrode. This type of circuit is referred to as a ground-return circuit, or sometimes an earth-return circuit.
An example of a ground-return surveying system is shown in FIG. 1. System 100 includes a transmitter 102, a first electrode 104 connected to the transmitter 102 via current wire 108 for driving an electrical current into the ground 150, and a second electrode 106 inserted into the ground at a spaced apart location from first electrode 104 and connected to the transmitter 102 via a second current wire 109. Current flows between first electrode 104 and second electrode 106 in the ground to complete the circuit.
The distance range of 50 m to 5000 m indicated in FIG. 1 between transmitter 102 and electrode 104 is only an example range. An electrode may be positioned more closely or farther away from a transmitter or another electrode at distances outside of this example range.
Such surveys are often conducted in a “pole-dipole” configuration where one electrode is located far from the survey area and is often called the “infinite” electrode. With the infinite electrode fixed, the second electrode is moved to different positions in the survey area while potential differences are measured between electrodes in an array of potential electrode pairs. The removal of the infinite electrode from the survey area typically improves the depth of search and simplifies the processing. Whatever the motivation, such surveys are often conducted with current to infinite electrode distances of several kilometers or more.
The potential difference measured between electrodes is a function of the resistivity structure of the ground and the injected current. The injected current is in turn directly proportional to the applied voltage and inversely proportional to the circuit resistance, which can be thought of as the series resistance of the ground circuit and the contact resistance of both current electrodes. Particularly in situations where the ground resistivity is low but the surface contact resistance is high, it is necessary to provide high voltages to drive sufficient current to in turn provide for sufficient data quality and survey speed. Transmitters capable of over 4000V are readily available and routinely used. Such high voltages can pose a significant hazard to crew members and the public at large.
The current wire connecting an electrode to a transmitter can consist of a single conductor, such as a stranded copper wire insulated with a polyethylene jacket. The wire is usually deployed on the ground from small reels running along routes of convenience such as along cut lines, paths, roads, etc. Because of the large distances, it is usually necessary to cut and splice various sections of the current wire together to form a single run.
While operating the transmitter, the current wire can pose a significant hazard to the crew and the public at large as well as to wildlife and domesticated livestock. Voltages as high as several thousand volts can exist between the wire and ground beneath it. While the current wire is insulated with a wire jacket, splices, cracks and other breaches of the insulation can allow such high voltages to leak current to ground. A leak to ground is referred to as a “ground fault”. The risk of a ground fault is usually higher when the ground and wire are wet. The problem is most acute when a person is holding or otherwise touching the wire and their feet are in contact with wet ground since a grounding path exists through the arm and across the heart of the person. In such a case, a leakage current as low as 5 milliampere (mA) is considered hazardous.
Hazard mitigation efforts for resistivity surveys usually include posting signage alerting the crew and public to the high-voltage hazard, performing traffic control into the survey area, and implementing communication protocols for ensuring that the crew is aware of the status of the transmitter at all times. Despite these efforts, significant risk remains to both the crew and the public. It is often impractical to control access to such a wide area covered by the current wire. Furthermore, wildlife and livestock sometimes chew current wires thereby breaching the insulation. In addition, people occasionally cut and steal the current wire for its use as wire or for its value as scrap copper. All of these activities expose people and animals to potentially fatal electric shock.